A number of lower olefins and di-olefins are known to be widely used in a variety of chemical processes, both as starting materials and as intermediates. These may include, in non-limiting example, ethylene, propylene, butene, iso-butene, and butadiene. While olefins and di-olefins may be by-products of some industrial processes, such as fluid catalytic cracking, the increasing need for olefins motivates development of “on-purpose” olefin and/or di-olefin production. One such “on-purpose” method is catalytic dehydrogenation of paraffins and/or other dehydrogenatable hydrocarbons.
In processes for the catalytic dehydrogenation, catalyst which has passed through the catalytic dehydrogenation reactor once or several times may still contain significant levels of activity. Such used catalyst which maintains some activity is referred to as used catalyst. Catalyst which maintains little or no activity is referred to as spent catalyst. Dehydrogenation catalysts are typically separated from a product stream after exiting the catalytic dehydrogenation reactor. Following such separation, all or part of the catalyst particles may be sent to regeneration. As some separated catalyst particles are used and maintain some activity, an economic benefit can result from recycling some of the separated catalyst. Moreover, dehydrogenation catalyst recycle may enable the catalyst feed temperature and the reactor space velocity to be controlled. It is beneficial to control the catalyst temperature in the reactor as too high temperatures results in poor selectivity. In addition, the reactor space velocity can be adjusted by way of a catalyst recycle stream thereby allowing process controllers to respond to a deactivating catalyst or potential miscalculations in the scale up.
In conventional fluid catalytic cracking systems, steam is used as a strip gas to remove any hydrocarbons entrained with the recycle catalyst. Steam is desirable in the catalytic dehydrogenation of paraffins because it will condense and easily be separated from hydrocarbons by forming a separate and distinct phase. However, steam severely deactivates the catalyst at relevant temperatures, as can be seen in FIG. 9, which illustrates an activity drop from 45% to 18%.
This disclosure addresses these issues by providing a process for recycling used dehydrogenation catalyst while maintaining catalyst activity and selectivity.